Measurement of Resonant Frequency of Resistance Sensor--Digital Waveform Generator

Variable resistor sensors can transform a fixed DC excitation voltage or current into a current or voltage that is a direct function of the measured value. In another type of sensor, a moving object or fluid can generate a sensor signal by changing the inductance or capacitance of an LC circuit. Figure 1 shows a basic AC-driven tuned circuit proximity sensor (i.e., L and C) and sampling resistor R. Under static conditions, L and C resonate and have maximum impedance at a certain frequency. When an object approaches the sensor, the value of L or C changes and changes the resonant frequency of the circuit. As long as the sensor is excited with a fixed frequency and the phase or amplitude change of the output voltage V2 relative to the excitation voltage V1 is measured, the position of the object can be derived. However, this method limits the dynamic range and resolution of the sensor.

As an alternative, a swept-frequency AC source that tracks the sensor's resonant frequency can be used to drive the sensor. Figure 2 shows a method where a DDS (direct digital synthesis) device IC1 provides a sine wave excitation voltage. Low-pass filter IC2 removes clock noise and harmonics. Amplifier IC3 drives the sensor. Amplifier IC4 boosts the sensor's output voltage V2 and drives IC5. IC5 is a dual-channel 12-bit ADC that samples and digitizes both reference voltage V1 and IC4's output. Microcontroller IC6 with DSP functionality analyzes the amplitude and phase of the sensor output and sets IC1's frequency by alternately programming a pair of IC1's frequency control registers. A serial port on IC6 transmits position data to an external controller. Using a DDS/DSP combination can provide considerable flexibility when using different types of sensors. For example, some sensors require a relatively narrow excitation frequency range with high resolution, while others may work best with wide swept-frequency excitation.